In order to stably operate a high-intensity ultrashort pulse laser device, as disclosed in Non-Patent Document 1, it is important that a pulse width of a light pulse is expanded to a chirped pulse by a pulse width conversion device (pulse expanding device) before optical amplification, an instantaneous intensity of the light pulse is suppressed to a low level in an optical amplifier device, and the pulse width of the light pulse is compressed by the pulse width conversion device (pulse compressing device) after the optical amplification, to increase a peak value of the light pulse. Such a method is called a chirped pulse amplification method.
A chirped pulse is a light pulse having a property in which an arrival time differs according to each of wavelength components included in the light pulse. The lower limit of the pulse width of the light pulse is determined according to a bandwidth of a wavelength band composing the light pulse. This is called Fourier-transform-limited pulse width. A pulse width of a chirped pulse is longer than the Fourier-limited pulse width. However, a chirped pulse is caused to pass through a device in which optical path lengths of the respective wavelength components composing the pulse are adjusted to be predetermined lengths, to be able to compress its pulse width into approximately the Fourier-limited pulse width.
The pulse compressing device is generally a device which is capable of compressing the above-described chirped pulse into approximately the Fourier-transform-limited pulse width. By arranging the pulse compressing device at the final stage of the high-intensity ultrashort pulse laser device, it is possible to compress a pulse width of a high-energy chirped pulse which has been amplified, to make the pulse width as short as possible, which makes it possible to increase a peak value of the light pulse. Here, depending on adjustment, it is possible to output a light pulse with a time width longer than the Fourier-transform-limited pulse width.
On the other hand, such a pulse compressing device is capable of operating as a pulse expanding device that expands a pulse width of a light pulse to a chirped pulse. Such a pulse width conversion device (a pulse compressing device, a pulse expanding device) that converts a pulse width of a light pulse includes some kind of spectroscopic element as an essential component. As spectroscopic elements, there are mainly elements such as prisms utilizing substance-specific dispersion and elements such as diffraction gratings utilizing the diffraction effect due to its device structure.
Pulse width conversion devices including prisms as spectroscopic elements have a narrow variable range for a pulse width of a light pulse, and therefore, it is difficult to apply those to the chirped pulse amplification method. Then, pulse width conversion devices including diffraction gratings as spectroscopic elements have been widely used. FIG. 12 to FIG. 15 are diagrams showing configuration examples of pulse width conversion devices including diffraction gratings as spectroscopic elements.
A pulse width conversion device 2A shown in FIG. 12 includes four reflection-type diffraction gratings 31 to 34. In the pulse width conversion device 2A, an input light pulse Pi is diffracted to be dispersed by the reflection-type diffraction grating 31, and is diffracted to be a parallel beam of light rays by the reflection-type diffraction grating 32, and is diffracted to be converged by the reflection-type diffraction grating 33, and is diffracted to be coupled by the reflection-type diffraction grating 34, and is output as an output light pulse Po.
A pulse width conversion device 2B shown in FIG. 13 includes four transmission-type diffraction gratings 21 to 24. In the pulse width conversion device 2B, an input light pulse Pi is diffracted to be dispersed by the transmission-type diffraction grating 21, and is diffracted to be a parallel beam of light rays by the transmission-type diffraction grating 22, and is diffracted to be converged by the transmission-type diffraction grating 23, and is diffracted to be coupled by the transmission-type diffraction grating 24, and is output as an output light pulse Po.
Conventionally, the configuration of the pulse width conversion device 2A including the four reflection-type diffraction gratings 31 to 34 as shown in FIG. 12 has been mainly used. However, in comparison to the reflection-type diffraction gratings, the transmission-type diffraction gratings are thermally superior due to their low optical absorption, and are further superior in terms of price due to the production process. For this reason, in recent years, the configuration of the pulse width conversion device 2B including the four transmission-type diffraction gratings 21 to 24 as shown in FIG. 13 has been used. Further, as shown in FIG. 14 and FIG. 15, there are also configurations of pulse width conversion devices 2C and 2D including two transmission-type diffraction gratings.
The pulse width conversion device 2C shown in FIG. 14 includes the two transmission-type diffraction gratings 21 and 22. In the pulse width conversion device 2C, an input light pulse Pi is diffracted to be dispersed by the transmission-type diffraction grating 21, and is diffracted to be a parallel beam of light rays by the transmission-type diffraction grating 22, and its optical path is folded back by a rectangular prism 40. The light pulse whose optical path is folded back by the rectangular prism 40 is diffracted to be converged by the transmission-type diffraction grating 22, and is diffracted to be coupled by the transmission-type diffraction grating 21, and is output as an output light pulse Po.
The pulse width conversion device 2D shown in FIG. 15 as well includes the two transmission-type diffraction gratings 21 and 22. In the pulse width conversion device 2D, an input light pulse Pi is diffracted to be dispersed by the transmission-type diffraction grating 21, and is reflected by reflecting mirrors 41 and 42 in series, and is diffracted to be a parallel beam of light rays by the transmission-type diffraction grating 22, and its optical path is folded back by a rectangular prism 40. The light pulse whose optical path is folded back by the rectangular prism 40 is diffracted to be converged by the transmission-type diffraction grating 22, and is reflected by the reflecting mirrors 42 and 41 in series, and is diffracted to be coupled by the transmission-type diffraction grating 21, and is output as an output light pulse Po.
In FIG. 12 to FIG. 15, a direction in which the lattices of the respective diffraction gratings are extended is a direction perpendicular to the plane of paper, and a light pulse travels parallel to the plane of paper except for the time of folding back the optical path by the rectangular prism 40. The rectangular prism 40 reflects the light pulse by the two reflecting surfaces in series to move the optical path of the return light pulse parallel in the direction perpendicular to the plane of paper with respect to the optical path of the input light pulse.
As shown in FIG. 12 to FIG. 15, in general, the pulse width conversion devices need light pulse incidence onto their spectroscopic elements several times. The number of light pulse incidences onto the spectroscopic elements is four at minimum. In contrast to the pulse width conversion devices 2A and 2B shown in FIG. 12 and FIG. 13, the pulse width conversion devices 2C and 2D shown in FIG. 14 and FIG. 15 allow incidence of light pulses onto the respective diffraction gratings twice, so as to reduce the number of diffraction gratings. Further, the pulse width conversion device 2D shown in FIG. 15 can be configured by one long diffraction grating into which the diffraction gratings 21 and 22 are integrated.